During the late Cretaceous, as sauropods and theropods roamed what would later
become Utah and Wyoming, the slab of oceanic crust that had been subducting below
western North America for the previous 100 million years began to shallow in its
trajectory beneath the continent. This ushered in a new era of subduction tectonics
in the region commonly referred to as that of the Laramide Slab. The shallow,
subducting Laramide Slab left its own fossil footprint on the land. Now, Jason
Saleeby of Caltech says the Laramide Slab owes its uniqueness to having carried
a plateau from the distant seafloor, which helps explain why the shallow oceanic
slab sheared off a segment of western North Americas continental lithosphere.

Geologists in the 1970s
and early 1980s first identified the Laramide Slab as a significant factor in
the formation of the Rocky Mountains and Colorado Plateau. Through geophysical
methods, including testing the depths of the regions continental roots with
seismic profiles, they have now determined that the Laramide Slab sheared off
perhaps up to 100 kilometers of the base of the continental lithosphere along
the southern Sierra Nevada and Mojave Desert region. In doing so, it replaced
the sheared-off lithosphere with deep sea-trench deposits that were subducted
along with the shallow slab. It also uplifted and denuded the deep continental
basement rocks of the region. Such deep-level basement rocks and schists that
were metamorphosed from the subducted sediments are visible today in the Tehachapi
Range and Rand Mountains.

Jason Saleeby (standing) with his Caltech
geology field class in the southern Sierra Nevada. The skyline ridge in the center
background is currently being investigated as a lower plate remnant of the breakaway
zone for the unloading of upper crustal fragments that were shed off the southern
Sierra Nevada during the Laramide orogeny. Photo by Ryan Petterson.

In the late 1970s, geologists recognized that in order to push and scrape that
much of the lithosphere while so far inland from the subducting trench, the Laramide
Slab must have been shallow as it went down. Reporting in the June GSA Bulletin,
Saleeby suggests a mechanism for such a shallow slab trajectory: The Laramide
slab carried with it a plateau of volcanically thickened seafloor. This plateau,
Saleeby explains, contained about 15 to 20 kilometers of oceanic crust basalt
rather than the usual 5 to 6 kilometers of basalt seen in typical abyssal crust.
It not only formed a bathymetric high on the seafloor as it entered the subduction
zone, but it also rendered the hosting slab more buoyant, contributing further
to its shallow subduction. The slab ultimately scraped and pushed the bottom of
the continental lithosphere on its way down to the mantle, creating a mess of
mountains at the surface. We see that going on today in the Andes where
active subduction is taking place, Saleeby says.

Sediments and fossils observed today in the subduction accretion assemblages exposed
in the California Coast Ranges host similar traits (particularly fossil assemblages)
to those that are intact on the Hess-Shatsky Rise, a large igneous province in
the Pacific Ocean, just west of the bend in the submerged Hawaiian-Emperor Seamount
chain, Saleeby says. He interpreted this fossil evidence, along with plate histories,
to mean that the sediments actually originated from the same place. The Hess-Shatsky
Rise, Saleeby suggests, once straddled a mid-ocean ridge, much like Iceland does
today, and thus was split during and subsequent to its formation so that one part
traveled west toward Asia and the other east toward North America. The portion
heading west will sink below Japan and the adjacent Bonin-Mariana Islands in another
50 million years or so. The half of the rise that headed toward North America
added thickness and buoyancy to the Oceanic Plate and became the Laramide Slab
when it subducted below the continent during the late Cretaceous to early Paleogene.

A tour of the Sierra Nevada Mountains, Saleeby explains, reveals northern rocks
exposed from a shallow depth and the southern rocks originating from deeper down.
While the slab was traveling northeast, the shallow slab segment sloped uphill
to the southeast. Upon restoring the Southern California continental basement
to its pre-San Andreas fault and pre-Basin and Range configuration, one can see
the Laramide damage zone to the continent migrate northeastward from the continent
edge to its Rocky Mountain interior as the shallow slab segment progressed beneath
western North America, Saleeby says. Along their edges the shallow
slab segments warp into a more normal geometry. Saleeby and his students
have shown in previous reports that the lithosphere to the north, close to Yosemite,
remained intact down to 125 kilometers, while it was sheared off by the shallow
slab segment to the south. He based those studies on samples from Miocene volcanic
eruptions, which had formed since the time of the Laramide orogeny, bringing fragments
of the lithosphere, in the form of xenoliths, to the surface.

Says geophysicist Eugene Humphreys of the University of Oregon: I think
the flat slab prepared the base of North America lithosphere beneath the Rocky
Mountains and Basin-and-Range area, which enabled the amazingly active post-Laramide
magmatism to occur. It is the post-Laramide magmatism that we mostly see in the
heat flow, seismic structure and strength fields today. Jason [Saleeby]s
ideas fit into this perspective well.